1. Field of the Invention
The invention relates to a method for preventing or retarding the formation of gas hydrates or for reducing the tendency of such hydrates to agglomerate during the transport of a fluid comprising water and a hydrocarbon through a conduit, and, more particularly, to the addition to the fluid of a gas hydrate inhibitor which is a copolymer of vinyl caprolactam and N,N-diethylaminoethyl(meth)acrylate of defined composition, which is preferably made and applied in a suitable solvent.
2. Description of the Prior Art
It is well known in the art that the formation of gas hydrates in a conduit, e.g. a pipeline, during the transport of liquids, such as oil, and gases, particularly lower hydrocarbons, e.g. methane, ethane, propane, butane, isobutane and natural gas is a serious problem, especially in areas with a low temperature in the winter season or in the sea. Generally the temperatures are so low that gas hydrate formation, due to the inevitable presence of co-produced water in the wells takes place, if no special steps are taken. Insulation decreases the chance of gas hydrate formation; however, if the field is relatively small and far away from the production platform, the costs of using insulation are too high to make such field economically attractive. It is also known to add anti-freeze compounds, for example, glycol or methanol, during transport to minimize gas hydrate formation; however, large quantities of these compounds are required to be effective which is expensive. Alkyl glycosides also have been used for this purpose.
A representation of the prior art in this field are the following U.S. Pat. Nos. 4,915,176; 5,420,370; 5,432,292; 5,723,524; 5,741,758 and 6,117,929; EPA 0526929A1; EPO 0323774A1; Can. Pat. Appln 2,073,577; WO 93/25798; WO 95/17579; Gas Hydrates and Hydrate Prevention 73 GPA Annual Convention, pgs 85-93; WO 96/08456; WO 96/08636; WO 93/25798; EPA 0457375A1 and WO 9412761.
What is described herein is a useful composition for effectively preventing or retarding the formation of gas hydrates, or for reducing the tendency of gas hydrates to agglomerate, during the transport of a fluid comprising water and a hydrocarbon, through a conduit, which includes a copolymer of vinyl caprolactam (VCL) and N,N-diethylaminoethyl(meth) acrylate (DEAEMA), preferably 80-95 wt. % of VCL and 5-20 wt. % DEAEMA. Most preferably, the copolymer is made and applied in a low molecular weight glycol ether, usually 2-butoxyethanol (BGE).
Generally, the gas hydrate inhibitor of the invention, i.e. copolymer of 80-95 wt. % VCL and 5-20 wt. % DEAEMA, is present in an amount of about 30 to 50% by weight of the composition, i.e. in admixture with the solvent. The product inhibition concentration in the pipeline, i.e. in the aqueous phase, is about 0. 1 to 3% by weight. The solvent inhibition concentration, accordingly, is about 0.1 to 5% by weight in the aqueous phase.
Suitable solvents include low molecular glycol ethers containing an alkoxy group having at least 3 carbon atoms, N-methylpyrrolidone (NMP), ethylene glycol, water and blends thereof. Representative glycol ethers include 2-butoxyethanol (ethylene glycol monobutyl ether); propylene glycol butyl ether; (diethylene glycol) monobutyl ether; and 2-isopropoxy-ethanol. 2-Butoxyethanol (BGE) is preferred.
Preferably the copolymers are made in the desired solvent and maintained therein in the composition of the invention. Less preferably, they are made in another solvent, such as isopropanol solvent, the solvent removed, and the desired glycol ether solvent added.
Most preferably, the copolymer is made in BGE at about 50% solids and then ethylene glycol is added with water as solvent.
The gas hydrate inhibition tests were conducted in a 500 ml, 316 stainless steel autoclave vessel having a usable volume of 200 ml, equipped with a thermostated cooling jacket, sapphire window, inlet and outlet, platinum resistance thermometer (PRT) and magnetic stirring pellet. The rig is rated up to 400xc2x0 C. and down to xe2x88x9225xc2x0 C. Temperature and pressure are data logged, while the cell content is visually monitored by a boroscope video camera connected to a time lapsed video recorder. Hydrate formation in the rig is detected using a combination of three methods: visual detection of hydrate crystals, decrease in vessel pressure due to gas uptake and by the temperature exotherm created by heat released during hydrate formation.
The rigs were thoroughly cleaned after each test. Each rig was initially rinsed 3 times each with 500 ml tap water after the test fluid was removed. An air drill fitted with a 3M Scotch-Brite cylindrical scouring pad was then used to remove traces of any adsorbed chemicals therein with a small amount of water added to the rig. Each rig was then rinsed with 500 ml double distilled water 2 more times and dried. A blank solution of 200 ml double distilled water was run to confirm test reproducibility. The formation of hydrates within 4-15 minutes was taken as a standard time for a given set of testing conditions, i.e. synthetic gas, 35 bar, and 4.5xc2x0 C. The cleaning procedure was repeated until a reproducible time was achieved.
Synthetic gas mixture of the following composition was used for hydrate testing:
200 ml of pre-cooled polymer solution (with total concentration equal to 0.5 wt %) was placed into the vessel, followed by a PTFE stirrer pellet. The rig top was replaced and the securing ring tightened. The boroscope and video camera were then attached to the apparatus. The rig was then stirred and allowed to cool to the required temperature. Upon reaching the pre-set temperature, the stirrer was stopped and the video recorder and computer data logger started. The rig was then charged with synthetic gas to reach the required pressure. A slightly higher pressure (2-3 bars) was used to allow for some gas dissolution in the water and the slight drop in the pressure as the gas cools. The stirrer was started at 500 rpm and the temperature (4.5xc2x0 C.), pressure (75 bar) and start time (t0) recorded. The run was terminated upon the formation of hydrates, usually at the moment of a pressure drop, which might or might not follow the exotherm and visual hydrates formation depending on the amount of the hydrates formed and the amplitude of the effect. The final temperature, pressure and time (t) of the hydrates formation were noted.
The onset of the hydrate formation time=t-t0 (mins) is indicated in the examples given below. The relative efficiencies of the inhibiting polymers are thus proportional to the measured induction times.
Based on data obtained using PVTsim, a commercial hydrate equilibrium prediction package, the equilibrium melting temperature for hydrate decomposition for the synthetic gas in double distilled water and P=75 bar is about 19.0xc2x0 C., the hydrate sub-cooling is equal to 14.5xc2x0 C. (T=4.5xc2x0 C. is the temperature of the measurements).